The effect of miniscrew taper morphology on insertion and removal torque in dogs

Purpose: This study compared the insertion and removal torque of tapered and cylindric orthodontic miniscrews. Materials and Methods: Ninety-six miniscrews were placed into the buccal alveolar bone of the mandible in six male beagle dogs. Maximum insertion torque was measured during placement of 48 miniscrews, and the maximum removal torque was recorded after 3 and 12 weeks of loading. After 4 months of healing, 48 additional miniscrews were implanted in almost the same place as the first set of implants; insertion torque was again measured. Histologic analysis was performed after 3 and 12 weeks of loading. Bone-implant contact and bone volume/total volume were compared during each period and in each group. Results: The tapered miniscrews showed a higher mean maximum insertion torque (22.3 ± 3.2 Ncm) than the cylindric miniscrews (13.6 ± 2.0 Ncm) (P < .001). The mean maximum removal torque of the tapered miniscrews (9.1 ± 2.3 Ncm) was significantly higher than that of the cylindric miniscrews (5.7 ± 2.2 Ncm) at 3 weeks after placement (P < .05), but there was no significant difference in the mean maximum removal torque value between the tapered (4.2 ± 1.4 Ncm) and cylindric (3.0 ± 1.1 Ncm) implants after 12 weeks of loading. The percentage of bone-implant contact was similar between the groups after 3 weeks of loading and increased later. The percentage of bone volume/ total volume was higher in the tapered miniscrews than in the cylindric miniscrews after 3 weeks of loading (P < .05), but there was no significant difference between the groups after 12 weeks of loading. Conclusions: In this dog model, the tapered miniscrew design increased initial stability until 3 weeks of loading but had no additional effect on the secondary stability (ie, after 12 weeks of loading) versus a cylindric miniscrew design.